Summary: Calculated pKEnol Values for Enols of Carboxylic Acid Derivatives
HCdC(OH)X (X ) OH, NH2, NMe2, OMe, OCHO, F, Cl, Br)
StepaŽn SklenaŽk,, Yitzhak Apeloig,*,, and Zvi Rappoport*,,§
Contribution from the Department of Chemistry, TechnionsIsrael Institute of Technology, Haifa 32000,
Israel, the Lise Meitner-MinerVa Center for Computational Quantum Chemistry and the Department of
Organic Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel
ReceiVed February 2, 1998. ReVised Manuscript ReceiVed July 22, 1998
Abstract: The energies of acetic acid and its CH3C(dO)X derivatives and of the corresponding enols H2Cd
C(OH)X, for X ) OH, NH2, NMe2, OMe, OCHO, F, Cl, and Br, were calculated by the MO ab initio method
MP2(full)/6-31G**, by single-point CCSD(T)(full)/6-311G**/MP2(full)/6-31G**), and by the hybrid density
functional method B3LYP/6-31G**. The calculated pKEnol ) -log KEnol values for the "keto"/enol equilibria
are all high (17.5-24.0) and follow the order for X:H < alkyl < OCHO < BrCl < F < NH2 < NMe2 <
OH, OMe. By using the appropriate isodesmic reactions, it was shown that all substituents stabilize more the
acid species than its enol (where Br and Cl are slightly destabilizing). All the computational methods display
a similar trend. The structures and energies of the various conformations of both species are given. Comparison
with the scarce experimental pKEnol data shows a reasonable agreement with the calculations.
Introduction
Whereas the equilibria and tautomerization kinetics of the
keto/enol system was extensively investigated,1 little is known
about the enols of carboxylic acid derivatives, R1R2CHCOX,